A numbers game
This sentence is grammatically correct:
A number of movies are screening tonight.
But if you stare at it for too long, you might start to wonder why. Why do we say “a number of movies are screening” rather than “a number of movies is screening”? After all, “number” is singular – surely the verb that follows it should be singular too? We would never write this:
A box of tissues are sitting on the coffee table.
The verb phrase should clearly be “is sitting”, to agree with the singular “box”. But isn’t the movies sentence structurally identical to the tissues sentence: a + singular noun + of + plural noun? Well, not exactly. Let’s find out why.
We know that in a basic sentence, the verb is the action and the subject is the thing doing the action. The verb must always agree with the subject – that’s why we say “I play the piano” but “she plays the piano”. The problem with the movies/tissues sentences is that we can’t immediately tell what the subjects are. In order to work that out, we need to discuss determiners.
A determiner is a word or phrase that comes before a noun and puts that noun into context. There are many types of determiner, but today we’re focusing on quantifying determiners (also called quantifiers). These are words that indicate the quantity of the noun, such as “some”, “all”, “many”, “every”, “most”, “few”, “several” and “no”. Quantifying determiners can also be phrases, such as “a lot of”, “plenty of”, “the majority of” and – you guessed it – “a number of”. Here’s the important part: a determiner cannot itself be the subject of the sentence; it can only come before the subject and put it into context by describing its quantity.
The movies sentence
Now that we know “a number of” is a determiner, we can work out the structure of the movies sentence more easily:
A number of [determiner] movies [subject] are screening [verb phrase] tonight [adverb].
The verb “are” is correct because it agrees with the subject, the plural “movies”. If you’re still finding it hard to see “a number of” as one unit, try mentally replacing it with another quantifying determiner, such as “many”. Clearly, it would be wrong to say this:
Many movies is screening tonight.
It doesn’t matter that “a number of” is a phrase and “many” is just a word – they both perform the same role in the sentence.
There is/there are
Things get a bit more confusing when the word “there” is introduced. For example, we could rephrase the movies sentence like this:
There are a number of movies screening tonight.
If you’re on the hunt for a standard subject + verb construction, you might be fooled into thinking “There” has now replaced “movies” as the subject. But “there” is what’s known as a dummy subject – it appears next to the verb and stands in for the real subject, which appears elsewhere in the sentence. The real subject of this sentence is still “movies”, and the verb should still agree with that.
The tissues sentence
We can now revisit the tissues sentence and understand why it uses “is” instead of “are”. Unlike the movies sentence, this one doesn’t contain any determiners. Instead, it has a modifier, “of tissues”, placed after the noun “box” to give more information about it:
A box [subject] of tissues [modifier] is sitting [verb phrase] on the coffee table [adverb phrase].
There you have it. It's really just a coincidence that “a number of movies” and “a box of tissues” share the same a + singular noun + of + plural noun structure, because they are completely different types of phrase.
In a nutshell
The phrase “a number of” is a quantifying determiner. It may be placed before the subject of a sentence, but it cannot itself be the subject. Therefore, in the phrase “a number of movies”, the subject is “movies”, and the verb that follows should be the plural “are”.
In sentences such as “There are a number of movies screening tonight”, “There” is a dummy subject; “movies” is still the real subject and the verb should still agree with it.